Chris Froome strengthened his grip on the yellow jersey despite being docked 20 seconds for an illegal feed, as Christophe Riblon won Stage 18 of the Tour de France to claim the host nation's first victory in the 100th edition of the race.

The penalty was given after Froome called for an energy bar when struggling with the closing kilometres of the famous Alpe-d'Huez climb - receiving food towards a stage finish is against the race rules.

For the first time in Tour history, riders climbed the peak twice in order to celebrate the centenary year. It did not disappoint.

Just 24 hours after claiming his third stage win of this year's Tour, Froome finished 57 seconds ahead of main rival Alberto Contador - even after the penalty - in the 172.5km stretch from Gap to Alpe-d'Huez, moving clear of the Saxo-Tinkoff rider on the second climb of the mountain.

In a tense finish, Riblon closed down a 45 second gap on the second half of the final ascent to snatch victory from a tiring Tejay van Garderen, with just two kilometres remaining.

Richie Porte had earlier provided supreme support to Team Sky rider Froome, while Nairo Quintana of Movistar cut the overall gap to the race leader by one minute and six seconds to move into third in the general classification.

For the first time in two and a half weeks, Froome began to show signs of a struggle and soon called for Porte to collect an energy bar from the Team Sky support car. However, race officials later decided that Froome had indeed broken the rules, issuing a time penalty after the 28-year-old had crossed the line.

"It was a difficult stage," Froome said. "When I signalled for help I needed sugar and I was thankful that Richie was there."

Joaquim Rodriguez of Katusha edged Porte and Froome to finish fifth behind Quintana after a strong last climb. Manxman Mark Cavendish came in well behind the leaders, crossing the line in 146th over 30 minutes behind.

However the day belonged to Frenchman Riblon, who ended his country's wait for a stage win with a remarkable climb to catch van Garderen on the famous ascent.

After enduring a tricky start to his Manchester United career, perhaps it is fair that Marcos Rojo celebrated so boisterously as he watched his first professional club Estudiantes beat fierce rivals Gimnasia